Blood tests for allergen specific IgE (also known as RAST)
Immunoglobulin E (IgE) antibodies directed against specific allergens can be measured with a blood test, formerly referred to as RAST tests (RAST was the abbreviation for the original name of the technology – RadioAllergoSorbent Test). These tests are often performed when skin testing is not easily available, when skin conditions such as severe eczema preclude skin prick testing, or when a person is taking medications (such as antihistamines) that interfere with accurate skin prick testing.
Total IgE testing
Measurement of total IgE antibodies can be estimated from a blood sample. Total IgE may be raised in people with allergies. High total IgE antibody levels are also found in people with parasite infections, eczema and some rare medical conditions. High IgE levels do not prove that symptoms are due to allergy, and a normal IgE level does not exclude allergy. Therefore, measuring total IgE levels is not routinely recommended in allergy testing.
Eosinophil counts
Eosinophils are specialised white blood cells that are designed to kill worms and parasites. They can also cause tissue inflammation in allergy. High levels are sometimes seen in blood samples from people with allergic rhinitis (hay fever), asthma and eczema, as well as in a number of less common conditions. However, a high eosinophil count does not prove that symptoms are due to allergy, and a normal eosinophil count does not exclude allergy. Therefore measuring eosinophil counts has a limited role to play in allergy testing.